1. Field of the Invention
The present invention relates in general to a method and a device for controlling the thickness of a strip being rolled by a rolling mill, and more particularly to improvements in the strip thickness control method and device for the rolling mill.
2. Discussion of the Related Art
In a known rolling mill, the thickness of a strip being rolled is controlled by adjusting the roll gap which is defined by work rolls at a rolling stand. When the strip thickness is controlled by regulating only the roll gap, the tension of the strip, particularly the strip tension measured on the upstream side of the rolling stand tends to be varied. This variation in the strip tension adversely affects the thickness of the strip. Therefore, it has been considered necessary to regulate the strip tension, as well as the roll gap, for accurately controlling the strip thickness.
In view of the above, the conventional control system for a rolling mill is adapted to control both the thickness and the tension of the strip, independently of each other. For example, the strip thickness at a given (i)th rolling stand is controlled by adjusting the roll gap of that (i)th stand, based on a proportioned and integrated value of a variation of the strip thickness which is detected by a suitable detector such as an X-ray thickness gauge. On the other hand, the tension of the strip is controlled by adjusting the rolling speed of an upstream (i-1)th rolling stand, as viewed in the direction of movement of the strip through the rolling mill. An example of such a control arrangement for a rolling mill is indicated in FIGS. 5(a) and 5(b).
Described more specifically referring to FIG. 5(a) which schematically illustrates the conventional control process for controlling the thickness of a strip at a given (i)th rolling stand of a cold tandem rolling mill, reference numeral 2 denotes a hydraulically operated roll gap adjusting actuator or device, while reference numeral 4 denotes a rolling speed adjusting device. These two adjusting devices 2 and 4 are controlled independently of each other. The roll gap adjusting device 2 is provided at the (i)th rolling stand indicated at 6, while the rolling speed adjusting device 4 is provided for a preceding or upstream (i-1)th rolling stand 8. On the downstream or outlet side of the (i)th stand 6, there is provided a thickness gauge 10 such as an X-ray gauge. Further, a tension meter 12 for measuring the tension of the strip is disposed between the (i)th and (i-1)th rolling stands 6, 8.
A proportioned and integrated value of a thickness variation detected by the gauge 10 is fed back to control the roll gap adjusting device 2 for adjusting the roll gap of the (i)th stand 6. In the meantime, a proportioned and integrated value of a tension variation detected by the tension meter 12 is fed back to the rolling speed adjusting device 4 for adjusting the rolling speed of the strip. The block diagram of FIG. 5(b) shows control arrangements for obtaining a roll gap command value U.sub.s.sup.(i) and a rolling speed command value U.sub.v.sup.(i-1) for controlling the devices 2 and 4. In the block diagram, h.sub.f.sup.(i) represents a variation of the thickness of the strip detected by the thickness gauge 10 on the downstream side of the stand 6, while .sigma..sup.(i-1) represents a variation of the tension of the strip detected by the tension meter 12 on the upstream side of the (i)th stand 6. K.sub.p.sup.(i) represents a proportion constant, while K.sub.I.sup.(i) represents an integration constant. Reference numerals 14, 16, 18 and 20 designate gain setters, while reference numerals 22 and 24 designate integrators.
In the control arrangement indicated above, however, the rolling environment disturbances received by the rolling mill cause a variation of the strip tension, which in turn unnecessarily causes a change in the rolling speed. Therefore, it takes a considerably long time until the roll gap is properly adjusted, and this indicates inaccurate control of the strip thickness.
In the light of above drawback experienced in the prior art thickness control arrangement for a rolling mill, the present applicants proposed a highly accurate strip thickness control arrangement wherein the roll gap adjusting device and the rolling speed adjusting device are simultaneously controlled, as disclosed in laid-open publication No. 62-214818 of unexamined Japanese patent application, which was published on Sept. 21, 1987.
Described more particularly, the strip thickness control arrangement disclosed in the above-identified publication is adapted to control the roll gap adjusting device and the rolling speed adjusting device of a rolling mill, such that the rolling environment disturbances received by the rolling mill are classified into a first disturbance for which only the rolling speed should be compensated, a second disturbance for which only the rolling gap should be compensated, and a third disturbance for which both the rolling speed and the roll gap should be compensated, and such that the thus classified first, second and third disturbances are estimated based on detected values of variations of the strip thickness on the downstream side of an appropriate rolling stand, tension of the strip on the upstream side of the stand, and the roll force of the stand. Based on the estimated values of the first, second and third disturbances, the rolling speed and roll gap adjusting devices are simultaneously controlled so as to maintain the strip thickness at a predetermined value, irrespective of the disturbances.
In the proposed thickness control arrangement, only the thickness information measured by a thickness gauge disposed on the downstream or outlet side of the appropriate rolling stand is used for controlling the thickness of the strip on the same side of the stand. Further study and research by the applicants revealed that this thickness information was insufficient for accurate regulation of the strip thickness. Thus, the applicants found it necessary to improve their earlier proposed strip thickness control arrangement of a rolling mill.